The binding characteristics of gonadal receptor sites for LH and FSH were analyzed in particulate testis fractions by kinetic and equilibrium studies with radioiodinated gonadotropins. Computerized methods of analysis were applied to receptor binding data to permit accurate modeling of receptor-hormone interactions, and to incorporate the several correction factors which should be employed to derive binding constants for labile systems such as peptide hormone receptors. More sensitive and practical radioligand-receptor assays for LH/hCG and FSH were developed. These methods, together with a sensitive in vitro bioassay for LH, were used to evaluate the binding and activation properties of native and modified gonadotropins.